already encamped, in the question how inflammation and vascular paralysis are distinguished. Our case is also distinguished by the fact that the skin, protected by callus like the practiced muscles, now affords better service under similar circumstances. The callus, m particular cases, represents an improvement in the grasping organ. Formative stimulation also occurs in the muscles; the contents of the primitive bundle are moved to nucleation through local stimulation, yet. the advantageous stimulation by exercise seems to be almost entirely, or chiefly, of a nutritive sort.
In like manner as the skin fortifies itself against the repeated touch of hot bodies by means of local calluses, it adapts itself to the heat of the sun through erythema and a change consequent upon it which is accompanied with the development of pigment, although pigment favors the absorption of the sunbeams. The fact is, perhaps, connected with this, that it is advantageous to animals to have the side that is turned toward the light of a dark color. Hence, as Moseley observed on the Challenger, Echeneis remora has the belly dark, the back light. Heat from artificial sources of a relatively lower temperature, which is deficient in refrangible rays, has a remarkably different effect from sunlight. Workers by the fire are pale. It is still to be seen whether the electric light will take the place of the sunlight in its effect on the skin as it does in the case of plants.
Horny structure becomes unfit for its purpose with insufficient use. A remarkable example of this is the cessation of the growth of the hoofs of horses and cattle on the soft turfs of the Falkland Islands, mentioned by Darwin. On the other hand, the hoofs of horses harden on dry, stony soils, as Xenophon teaches in his school for horsemen; and colts brought up on such soils need no protection.
The so-called rider's bones, the exercise-bones, which have not become rarer since the introduction of the new armor and the modified drill, but have moved from the left to the right, may be considered as a kind of inner callus, the development of which affords a new exemplification of the Osteo-blasten theory. These bones hardly bring any advantage to their possessor, and can not be included among the instances of self-improvement through exercise. It would be too far fetched and groping in a too dark quarter for me to do more than mention here that Ludwig Fick believes that the well-adapted form of the joints may have been derived from exercises during the fetal period and the earliest days of life. Is it not possible that the splendid formation of the spongy bone-substance in the epiphyses, which was discovered by Hermann Meyer, and further investigated by Julius Wolff, depends on nutritive and formative stimulus in the direction of the greatest pressure and strain? The injurious effect of insufficient use is shown in this region by the non-growth of the teeth of rodents when they are fed on too soft food, or after the trigeminus has been cut.